Modern motor vehicles are equipped with networked control devices, between which a plurality of data relating both to the state of the electrical and also to the state of the mechanical components of the motor vehicle is transmitted. Pulse width modulation (PWM) has particularly proven its worth as a transmission method as a result of its simplicity and immunity to interference within motor vehicles.
One application is for instance the transmission of control information for fan or pump motors, in which a pulse width-modulated control signal is generated in a master control device and transmitted via a bus, frequently a single wire line, to a slave control device which is locally remote from the master control device. The control devices generally contain microcontrollers, which have a self-diagnostic function. In the event of a fault, the fault message can occur for instance from a slave control device to a master control device, such that the slave control device pulls the electrical potential on the line of the single wire interface down to earth potential during a specific period. As a result, the master control device, which constantly monitors the voltage level on the single wire line, identifies the fault notified by the slave control device. The length of time allows different faults to be encoded, so that the master control device can take actions which are tailored to the fault. Such an action can consist for instance in the entire system being put into a more secure state. If the master control device is networked in a superordinate bus, in a controller area network (CAN) for instance, it can signal the fault on a display device in the passenger compartment and/or can save it in a fault storage device in the motor vehicle for later diagnosis purposes.
The disadvantage here is that the initiative comes from the slave control device, so that if the slave control device fails, the master control device constantly assumes that the system is in a fault-free operating state. This fault recognition is inadequate and can have far-reaching consequences depending on the type of controller. If the fan motor in an engine cooling system fails, without corresponding actions being taken, this can result in overheating and in some instances in damage to the internal combustion engine.